In recent years, studies have been aggressively made to develop a white light-emitting diode in which a blue light-emitting element using a nitride-based compound semiconductor is employed as a light emission source. A white light-light emitting diode is lightweight, uses no mercury and has long life, and therefore, demands are expected to rapidly expand in the future. A most commonly employed method for converting blue light of a blue light-emitting element into white light is a method where, as described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2000-208815, a coating layer containing a fluorescent material capable of absorbing a part of blue light and emitting yellow light and a mold layer for mixing blue light of the light source with yellow light from the coating layer are provided on the front surface of a blue light-emitting element, and a pseudo-white color is obtained by mixing a blue color and a yellow color which are in a complimentary color relationship with each other. As for the coating layer, a mixture of cerium-activated yttrium-aluminum-garnet (YAG:Ce) powder and epoxy resin has been conventionally employed. However, in this method, for example, uneven distribution of the fluorescent powder contained or a fluctuation in the amount of fluorescent powder among individual light-emitting diodes readily occurs when applying the coating layer, and it has been pointed out that the light-emitting diode has color unevenness attributable thereto.
In order to avoid this problem, a method has been proposed in which a light-emitting function is imparted to the substrate itself on which a blue light-emitting element is formed, and does not use powder. For example, Japanese Unexamined Patent Publication (Kokai) No. 2003-204080 proposes a method where a nitride semiconductor layer comprising InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1) is formed on a substrate of which main plane is a (111) face of YAG:Ce fluorescent single crystal, and blue light emitted from the semiconductor layer is made incident directly on the substrate to cause emission of homogeneous yellow fluorescence from the substrate itself, whereby a homogeneous white color free from color unevenness is obtained only by a light-emitting chip without using a fluorescent powder-containing coating layer.
As another method not using YAG fluorescent powder, Japanese Unexamined Patent Publication (Kokai) No. 2000-082845 discloses a method of obtaining a white light-emitting diode by using a ZnSe single crystal. This is a method of imparting a self-activated (SA) light-emitting function to a ZnSe substrate, forming a ZnSe-based blue light-emitting diode on the substrate, and simultaneously effecting emission of blue light and emission of yellow light from the device, thereby obtaining a white color.
However, the actual performance of the white light-emitting diode using a substrate with a YAG (111) face described in Japanese Unexamined Patent Publication (Kokai) No. 2003-204080 above is not known. The reason is because the difference between the lattice spacing of the YAG (111) substrate and the lattice spacing of InxGa1-xN constituting a nitride semiconductor buffer layer formed thereon is large and a good-quality nitride semiconductor layer cannot be readily formed.
The white light-emitting diode using a ZnSe single crystal described in Japanese Unexamined Patent Publication (Kokai) No. 2000-082845 suffers from deterioration of the device and for the improvement thereof, which is needed to enhance the quality of the ZnSe substrate. In particular, reduction in the dislocation density is necessary for prolonging the life, and improvements such as optimization of the device fabricating process and change of the material are being studied at present. This is described, for example, in Tsunemasa Taguchi (supervisor), Hakusyoku LED Shomei System Gijutsu no Oyo to Showai Tenbo (Application and Future Perspective of White LED Lighting System Technology), page 170 (2003).
At the present time, a (0001) face of Al2O3 single crystal (sapphire) is widely employed as the substrate for an InGaN-based blue light-emitting diode, and its actual performance has a long history. Moreover, no problems have been reported for a device produced using the Al2O3 single crystal, along with a deterioration in a light-emitting device. Accordingly, in producing a white light-emitting diode by using light emission of the substrate itself, production is most preferably achieved by a method of fabricating a blue light-emitting element on an Al2O3 substrate. For realizing this production, as described above, light emission of the substrate itself is necessary, but a method of obtaining yellow light emission by injecting blue light to an Al2O3 single crystal has yet not reported.
An object of the present invention is to provide a substrate for light-emitting diodes, which uses no fluorescent powder, enables to form a good light-emitting diode element, less causes deterioration, transmits light of the light-emitting diode element, emits light by utilizing a part of the transmitted light, and allows the transmitted light and newly emitted light to be mixed and emitted, and a light-emitting diode using the substrate for light-emitting diodes.